1. Field of the Invention
The present invention relates to an active-driving organic EL light emission device (which may be referred to merely as an organic EL device hereinafter) having a thin film transistor (which may be referred to as a TFT). More specifically, the present invention relates to an organic EL device used suitably for display equipment and color displays for the people's livelihood and industries, and the like.
In the present specification, the description “EL” means “electroluminescence”.
2. Description of the Related Art
Conventionally, it is known a simple-driving organic EL light emission device which is simply driven by XY matrix electrodes to display an image (Japanese Patent Application Laid-Open (JP-A) No. 37385/1990, JP-A No. 233891/1991 and the like) as an organic EL light emission device (display).
However, in such a simple-driving organic EL light emission device, the so-called line sequential driving is performed. Therefore, if the number of scanning lines is several hundreds, required instantaneous brightness is several-hundred times larger than observed brightness so that the following problems arise.
(1) Since a driving voltage becomes not less than 2–3 times higher than a direct-current constant voltage, luminous efficiency drops or power consumption becomes large.
(2) Since the electrical current that passes instantaneously becomes several-hundred times larger, the organic luminous layer is apt to deteriorate.
(3) Since the electrical current is very large in the same manner as in the (2), a voltage-drop in the electrode wiring becomes large.
Thus, in order to solve the problems that simple-driving organic EL light emission devices have, various active-driving organic EL light emission devices, wherein organic EL elements are driven by TFTs (thin film transistors), are suggested (JP-A No. 122360/1995, JP-A No. 122361/1995, JP-A No. /153576/1995, JP-A No. 54836/1996, JP-A No. 111341/1995, JP-A No. 312290/1995, JP-A No. 109370/1996, JP-A No. 129359/1996, JP-A No. 241047/1996, JP-A No. 227276/1996, JP-A No. 339968/1999, and the like).
Examples of the structure of such an active-driving organic EL light emission device are shown in FIGS. 18 and 19. According to such active-driving organic EL light emission devices, it is possible to obtain advantages as follows: driving voltage is highly lowered, luminous efficiency is improved and power consumption can be reduced, as compared with simple-driving organic EL light emission devices.
However, the following problems (1)–(3) are caused even in active-driving organic EL light emission devices having advantageous as described above.
(1) The aperture ratio of their pixels becomes small.
In an active-driving organic EL light emission device, at least one TFT is fitted to each pixel on a transparent substrate and further a great deal of scanning electrode lines and signal electrode lines are disposed on the substrate to select appropriate TFTs and drive them. Accordingly, there arises a problem that when light is taken out from the side of the transparent substrate, the aperture ratio of the pixels (the ratio of portions that emits light actually in the pixels) becomes small since the TFTs and the various electrode lines shut off the light. For example, in an active-driving organic EL light emission device that has been developed recently, TFTs for driving organic EL elements at a constant current are disposed besides the above-mentioned two kinds of TFTs. Therefore, its aperture ratio becomes smaller and smaller (about 30% or less). As a result, dependently on the aperture ratio, the current density that passes through the organic luminous medium becomes large, causing a problem that the life span of the organic EL elements is shortened.
This matter will be described in more detail, referring to FIGS. 10, 11 and 18. FIG. 10 shows a diagram of a circuit for switch-driving the active-driving organic EL light emission device 100 illustrated in FIG. 18, and illustrates a state that gate lines (scanning electrode lines) 50 (108 in FIG. 18) and source lines (signal electrode lines) 51 are formed on the substrate and they are in an XY matrix form. Common electrode lines 52 are disposed in parallel to the source lines (signal electrode lines) 51. About each pixel, a first TFT 55 and a second TFT 56 are fitted to the gate lines 50 and the source lines 51. A capacitance 57 is connected between the gate of the second TFT 56 and the common electrode line 52 to hold the gate voltage at a constant value.
Therefore, an organic EL element 26 can be effectively driven by applying the voltage held by the capacitance 57 to the gate of the second TFT 56 shown in the circuit diagram of FIG. 10 and then attaining switching.
The plan view shown in FIG. 11 is a view obtained by seeing, along the plane direction, through switch portions and the like according to the circuit diagram shown in FIG. 10.
Thus, the active-driving organic EL light emission device 100 has a problem that when EL light is taken out from the side of lower electrodes (ITO, indium tin oxide) 102 side, that is, the side of a substrate 104 side, a TFT 106, a gate line 108, a source line (not illustrated) and the like shut off EL light so that the aperture ratio of pixels becomes small.
In an active-driving organic EL light emission device 204, as shown in FIG. 19, wherein a TFT 200 and an organic EL element 202 are arranged on the same plane, the TFT 200 and the like never block off EL light. However, its aperture ratio of pixels is further lowered, as compared with the active-driving organic EL light emission device 100 shown in FIG. 18.
(2) The sheet resistivity of upper electrodes is large.
In the case that light is taken out from the side opposite to the substrate, that is, the side of upper electrodes, the TFTs and the like do not shut off the light to keep the aperture ratio large. As a result, a high-brightness image can be obtained. However, when EL light is taken out from the upper electrode side, in order to take out the EL light effectively to the outside, it is necessary to form the upper electrodes from transparent conductive material. For this reason, the sheet resistivity of the upper electrodes exceeds, for example, 20 Ω/□, resulting in a serious problem at the time of using large-area display.
In the case that light is emitted, for example, at a brightness of 300 nit from the entire surface of an EL light emission device having a diagonal size of 20 inches (the ratio of length to breadth, 3:4), it is necessary to send a large current having a current of 3600 mA to the upper electrodes even if an organic luminous material having a high luminous efficiency of 10 cd/A (luminous power per unit amperage) is used in the organic luminous medium.
More specifically, the value of a voltage-drop based on the resistances of the upper electrodes is represented by Σnir and calculated on the following formula.Σnir=½×N(N+1)ir 
N: (the total number of pixels in the longitudinal direction)×½,
r: the ohmic value (Ω) of the upper electrode in each pixel, and
i: a constant current value (A) that flows through each pixel.
Therefore, if luminous efficiency, luminous brightness, the shape of the pixels and the sheet resistivity of the upper electrodes are set to, for example, 10 cd/A, 300 nit, 200×600 μm square, and 20 Ω/□, respectively, the pixel current value is 3.6×10−6 A. If the total number of the pixels in the longitudinal direction is set to 2000, drop-voltage in the longitudinal direction is 12V (½×1000×1000×3.6×10−6×20×⅓). This exceeds an allowable voltage range (10 V) for driving circuits which are driven at a constant current. Thus, it is difficult to emit light under the above-mentioned conditions.
In short, if the sheet resistivity of the upper electrodes is large, voltage-drop, particularly at the center of the screen, becomes large accordingly. As a result, a problem that brightness is remarkably lowered becomes apparent. Incidentally, the following is also attempted: amendment is made by using a circuit to make a current value (brightness) constant for each pixel. However, this attempt is insufficient.
(3) From the viewpoint of production, it is difficult to control the ohmic value of the upper electrodes.
It is known that in order to set the resistivity of the upper electrodes of an active-driving organic EL light emission device having a diagonal size of several inches to 10 inches to a low value, for example, 1×10−3 Ω·cm or less by using an ordinary material such as ITO or ZnO, it is necessary to set heating temperature to 200° C. or higher. However, heat-resistance of ordinary organic luminous media is 200° C. or lower. Thus, it is necessary to set the heating temperature to 200° C. or lower. Accordingly, the value of the resistivity of the upper electrodes cannot be controlled so that the value may exceed 1×10−3 Ω·cm. As a result, a problem that the sheet resistivity becomes a high value over 20 Ω/□ occurs. In the case that plasma is used for sputtering at the time of forming an oxide such as ITO or IZO on the organic luminous medium to form the upper electrodes, a problem that the organic luminous medium is damaged by the plasma also arises.
In light of the above-mentioned problems, the present invention has been made. Its object is to provide an organic active EL light emission device making it possible to increase the aperture ratio of respective pixels even if TFTs are disposed to drive organic EL elements, reduce the sheet resistivity of upper electrodes even if luminescence is taken out from the side of the upper electrodes, and display an image having a high brightness and a homogeneous brightness; and a method for manufacturing such an organic active EL light emission device effectively.